1. Field of the Invention
The present invention relates generally to a wide area cordless telephone system which can be coupled to public or private switched telephone networks, and more specifically to such a system which is able to effectively locate cordless units therein.
2. Description of the Prior Art
A wide area cordless telephone system is designed to serve a relatively wide business area such as occurs within a building, factory, etc., and which is previously divided into a plurality of small service zones. An access or fixed station is provided in each of the service zones for establishing communications between the public or private switched telephone network and a plurality of cordless (or mobile) units located therein.
Before turning to the present invention it is deemed advantageous to discuss a known technique with reference to FIG. 1 which schematically depicts the overall layout of a wide area cordless telephone system.
As shown in FIG. 1, twelve access stations 11-22 are located at spaced intervals and arranged respectively to define twelve divided service zones 11-22 (denoted by the same numerals as the corresponding stations merely for the sake of description), the boundaries of which are specified by the corresponding broken lines. It should be noted however, that each of the service zones 11-22 is in actual practice inevitably irregularly shaped due to varying electrical field strength of signals from the corresponding access unit. For the convenience of explanation only two cordless units 40 and 40' are illustrated and are shown as being located within the service zones 11 and 21 respectively, in this particular example. Each of the access stations 11 to 22 is coupled to a radio control unit (RCU) 42 which is in turn coupled to a service console 44 and to a private branch exchange 46. Cordless units in the telephone network system shown in FIG. 1, may access a public switched telephone network denoted by 48 via the private branch exchange 46.
In order to establish communications between the cordless units employed in the wide area cordless telephone system, it is absolutely necessary that the RCU 42 is able to exactly locate all of the cordless units. To this end, when the cordless unit (for example) issues a unit location registration request signal (frequently referred to merely as "request signal" for simplicity), a known technique deals with the request signal as follows.
It is assumed that: (a) the cordless unit 40 issues a request signal and (b) each of the access stations 11-22 has received the request signal from the unit 40 and relays the request signal to the RCU 42 after adding thereto an electrical field strength of the request signal received. The RCU 42 compares the electrical field strengths (viz., signal levels) applied via a plurality of the access stations 11-22, and determines the strongest electrical field strength (viz., the highest signal level) and determines the location of the cordless unit 40 based on this paramenter. In the event that the electrical field strength information applied from the access station 11 is the strongest, the RCU 42 responds by determining the location of the cordless unit 40 as being service zone 11.
Although it is not clear from FIG. 1, the boundaries of the service zones are practically very complex with this kind of cordless telephone system, and as such there exists a high possibility that all of the access stations 11 to 22 may receive the request signal from the cordless unit 40 as above mentioned. In this case, the RCU 42 is undesirably required to compare a large number of signal levels each time such a location registration request signal issues. This problem inherent in the prior art technique is extensively enhanced when the request signal is intermittently issued by each of the cordless units at relatively short time intervals (30 seconds for example).